One process for transferring a useful layer to a receiver substrate, known from the prior art and described in French Patent FR2860249, comprises the following steps:
a) providing a donor substrate comprising an intermediate layer located between a carrier substrate and a useful layer, the intermediate layer being suitable to become soft from a first temperature;
b) providing a receiver substrate;
c) assembling the receiver substrate and the donor substrate; and
d) carrying out a heat treatment on the receiver substrate and the donor substrate after step c), the heat treatment being carried out at a second temperature higher than the first temperature.
The intermediate layer may be a layer comprising a vitreous material.
The intermediate layer generally comprises at least one of phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG).
The intermediate layer is characterized by a first temperature, generally called its glass transition temperature, from which the intermediate layer becomes soft and is capable of plastically deforming.
In step d), carried out at a second temperature above the glass transition temperature, micro-bubbles or micro-cavities are formed in the intermediate layer.
The micro-bubbles and micro-cavities formed in step d) originate from chemical species present in the intermediate layer. The chemical species transform into gas in step d). Since the intermediate layer and the useful layer are sandwiched between the carrier substrate and the receiver substrate, the chemical species transformed into gas are then trapped in the intermediate layer.
Consequently, the chemical species transformed into gas coalesce via an Oswald ripening mechanism so that the intermediate layer becomes spongy, and thus forms a weak zone.
It is then enough to exert a force on the intermediate layer to separate the carrier substrate from the structure formed in the assembling step c), and thus transfer the useful layer to the receiver substrate.
However, this process is unsatisfactory.
Specifically, the donor substrate cannot undergo processing steps comprising a temperature increase liable to generate a weak zone in the intermediate layer before the assembly step c). This would have the consequence of degrading the useful layer before its transfer.
Such a thermal annealing step is especially carried out in order to relax a pre-existing stress in the useful layer by softening the intermediate layer. By way of example, the useful layer may be a layer of GaN.
One object of the invention is thus to provide a process allowing the integrity of the useful layer to be preserved.